We are discussing this big 3 era and not the other Celtics era (which all won multiple titles)
I don't know how you measure success. Because IMHO, success here is winning a title. Anything less, it is what it is - a failure.
The big three were put together to aim at one thing - to get championship banners. That is the measure of success here.
From 2008 until now, the big 3 era just produced 1 title. Heck they even developed another superstar (Rondo) so that's Big 3 + 1, whilst the Lakers got Kobe plus 2 big sidekicks and yet they've produced 2 titles.
If you form an all NBA superstars to the Olympics and just got silver and not gold - you will call that a failure. Because gold would be the measure of success.
Again, the only measure of success here is how many times you will get the championship banner. Not the number of times you get to the finals or playoffs.